The present invention relates to an overrunning clutch for use, for example, in the engine starter of an agricultural machine (lawn mower or sprayer).
FIG. 5 is a side view, in longitudinal section, of a conventional transmission system disposed between a starter motor and a crankshaft and including an overrunning clutch.
Referring to FIG. 5, numeral 52 is a starter motor. A drive gear 54 mounted on the output shaft of the starter motor 52. An idle gearing 56 has a large gear 56a and a small gear 56b are integral together. Numeral 58 is a reduction gear. Numeral 60 is a transmission sleeve or cylindrical body on which the reduction gear 58 is fixedly mounted. The large gear 56a and the small gear 56b in the idle gearing 56 are meshed with the drive gear 54 and the reduction gear 58, respectively. With this arrangement, power is transmitted from the starter motor 52 to the transmission sleeve 60 with the speed of rotation of the starter motor 52 being reduced through the gears.
The sleeve 60 is fit coaxially around the crankshaft 62. A shell-type overrunning clutch 64 is disposed between the sleeve 60 and the crankshaft 62. As shown in FIGS. 6 and 7, the overrunning clutch 64 includes an outer member or shell 66, a retainer 68, and a plurality of rollers 70. The shell 66 has a cylindrical portion 66a, and a pair of flanges 66b extending radially and inwardly from opposite ends of the cylindrical portion 66a. Cam surfaces 66c are formed in the inner peripheral surface of the cylindrical portion 66a and correspond in number with the rollers 70. The retainer 68 is made of synthetic resin and includes a pair of opposite annular rings 68a and 68a, and a plurality of column portion 68b axially extending between the rings 68a. A plurality of roller pockets 68c are defined between adjacent column portions 68b. The rollers 70 are rollingly disposed within the corresponding roller pockets 68c.
Each column portion 68b in the retainer 68 has an integral spring 68d. The spring 68d takes the shape of a bifurcated tongue adapted to urge the roller 70 in the roller pocket 68b in the direction in which the roller 70 is locked between the cam surfaced 66c and the crankshaft 62.
The shell 66 of the overrunning clutch 64 thus far constructed is press fit in the sleeve 60.
Operation of the overrunning clutch thus constructed is as follows.
To start up an engine, the starter motor 52 is first energized to rotate the sleeve 60 in the direction of an arrow a.sub.2 through the drive gear 54, the idle gearing 56, and the reduction gear 58. This causes the shell 66 and the retainer 68 to rotate in the same direction. The rollers 70 urged by the springs 68d are then moved in the direction in which the space between the cam surfaces 66c and the crankshaft 62 are narrower. After the rollers have finally been locked between the cam surfaces 66c and the crankshaft 62, rotation of the shell 66 is transmitted through the rollers 70 to the crankshaft 62. The crankshaft 62 is then rotated in the direction of an arrow b.sub.2 so as to start up the engine.
The starter motor 52 is stopped when the engine has been started. However, the crankshaft 62 is rotated at a higher speed in the direction of the arrow b.sub.2 by means of the engine. At this time, the sleeve 60 is stopped as it is connected through the reduction gear 58, the idle gearing 56, and the drive gear 54 to the starter motor 52 now stopped. The shell 66 and the retainer 68 are also stopped as the shell is secured to the sleeve 60. A frictional force is applied from the crankshaft 62 to the rollers 70 to move the rollers 70 within the roller pockets 68c in the direction of the arrow b.sub.2. The rollers 70 are moved in the direction in which the space between the cam surfaces 66c and the crankshaft 62 are wider, against the action of the tongue-like springs 68d. The rollers 70 are freely rolled in the space and separated from the cam surfaces 66c and the crankshaft 62. As a result, power is no longer transmitted from the crankshaft 62 to the shell 66 and thus, the starter motor 52.
It has been stated that when the crankshaft 62 is driven for rotation by the engine, the rollers 70 are moved in the direction in which the space between the cam surfaces 66c and the crankshaft 62 are narrower and is then free to race in the wide space. Although the rollers 70 move in that direction immediately after they came into contact with the crankshaft 62, they are moved in the direction in which the space between the cam surfaces 66c and the crankshaft 62 are narrower as soon as they race, and a pushing force is no longer applied from the crankshaft 62. This is because the springs 68d always urge the rollers 70 in the direction in which the space is narrower. Thereafter, the rollers are brought into contact with the crankshaft 62 and again, moved in the direction in which the space is wider. The rollers 70 are contacted with and separated from the crankshaft 62 in an intermittent manner. This is called a "dancing phenomenon". When the rollers 70 are moved in the direction in which the rollers 70 are locked, they become worn and hot due to friction. This also prevents smooth rotation of the crankshaft 62. Frictional heat results in an increase in the temperature of the rollers 70, the crankshaft 62 and the retainer 68. Seizing may occur when the temperature of these components is raised above a predetermined level.
If seizing occurs, the crankshaft 62 and the shell 66 are locked to give an impact to or cause damage to the transmission system.
The retainer 68 is easily damaged due to frictional heat, particularly in the case that the retainer 68 with the springs 68d is made of synthetic resin.
Each roller 70 can be held in such a position that it races if the spring 68 has a smaller biasing force. However, such a smaller biasing force fails to firmly lock the roller 70 and to transmit rotation from the sleeve 60 to the crankshaft 62 when the engine is to be started up.